1. Field of the Invention
The present invention relates to a multilayer circuit board well suited for high density mounting of electronic devices such as semiconductor devices and more particularly to a structure of a multilayer circuit board formed by laminating together a plurality of printed wiring boards.
2. Related Background Art
As the result of a recent trend towards realizing a greater degree of integration of semiconductor devices, the density of wiring patterns on printed wiring boards (PWB) for carrying such semiconductor devices has become increasingly higher and higher. For instance, lead less chip-types have become the main current in the actual mounting forms of various electronic devices including semiconductor devices as well as resisters and capacitors, and high density surface mounting techniques for directly soldering chip-type devices on the printed wiring of a PWB to realize a high density mounting on the PWB have already been developed.
With the advent of higher levels of mounting forms, there has been an increase in the thermal stress at the soldered bonded joints between electronic devices, particularly semiconductor chips and a PWB thus giving rise to a danger of increasing the rate of occurrence of cracking at the bonded joints. To overcome this danger, measures have been proposed to reduce the coefficient of linear expansion of a substrate constituting a PWB so as to be close to that of the semiconductor chips. For instance, a glass epoxy multilayer substrate FR-5, formed by laminating a plurality of epoxy resin impregnated glass fabrics made from T-glass fibers having a low coefficient of linear expansion and used as reinforcement material, has been commercially available and PWBs formed from these substrates have already been put in practical applications.
However, the PWB formed from such a substrate reinforced with T-glass fibers has a coefficient of linear expansion in the range from 7 to 10 ppm and this rate is still large as compared with the coefficient of linear expansion, i.e., 3 ppm, of silicon semiconductor chips, for example. In other words, even if such a PWB is used, a thermal stress caused within the PWB due to temperature variations suffering during the mounting of electronic devices by soldering or during the use cannot be reduced so much, and the danger of causing cracking in the soldered layer at the bonded joints between the silicon chips and the wiring on the PWB still remains unsolved. In this case, in order to prevent the occurrence of cracking in the soldered layer, it is rather necessary to effect an additional procedure of filling the gaps between the silicon chips mounted on the PWB and its surface with a resin to reduce the thermal stress.
Also, the effect of the glass fibers or the reinforcement material of the substrate does not practically act on the through-holes formed in the PWB and thus the local coefficients of linear expansion in the axial direction of the through-holes show large values ranging from 50 to 150 ppm. Thus, due to the thermal stress caused by variations in the ambient temperature, there is the danger of causing a damage leading to any electrical break in the copper plating applied into the through-holes or the conductive paste (e.g., copper paste) filling the through-holes.
On the other hand, the formation of through-holes for providing electric connections between the respective layers is essential in the case of a multilayer printed wiring board (MPWB). Conventionally, a drill is used to form holes through the substrates for the formation of the required through-hole portions, and this method not only increases the number of drilling operations with an increase in the number of holes thus requiring much labour and time, but also has the danger of causing a gap due to separation at the substrate resin/glass fiber interface inside any through-hole due to the shock of drilling operation and causing the migration of copper ions from the copper plating layer formed on the through-hole inner surface into the gap, i.e., a so-called migration phenomenon. Such migration must be avoided by all means since it results in the deterioration of the electrical insulation performance between the respective through-holes or between the through-holes and other printed wiring.
According to the series of processing steps hitherto used for the fabrication of an MPWB, a plurality of double-sided printed wiring boards are first prepared and, after the double-sided printed wiring boards have been laminated together and subjected to the application of pressure at once, one or more through-holes are formed in this multilayer structure and a plating layer is formed on the inner surface of the through-holes. In this case, while the laminating process can be effected efficiently by pressing the plurality of double-sided printed wiring boards as a whole, the following through-hole forming procedure is inevitably required to collectively form the desired through-holes through the whole layers and the positions of the through-holes at the respective layers cannot be arbitrarily selected independently. Thus, there is the disadvantage of deteriorating the degree of freedom in wiring designing.
Also, in order to ensure improved electromagnetic noise shielding function and intercicuit crosstalk withstanding performance, it is desirable as one countermeasure to provide shielding layers between the respective layers of an MPWB. However, simply adding shielding layers between the respective layers only results in a considerable increase in the number of constituting layers as a whole and the resulting increase in cost cannot be avoided.
In consideration of applications in the high frequency range, the wiring formed on a PWB must be designed as transmission lines. In the case of the conventional PWB or MPWB, however, the provision of ground layers between the respective layers tends to considerably increase the number of wiring layers and this is disadvantageous from the standpoint of economy. Despite this fact, the absence of such ground layers tends to make it difficult to provide the desired impedance matching of the transmission lines and this eventually makes it extremely difficult to supply PWBs or MPWBs which are well suited for use in high frequency applications and are low in cost.
Attempts have already been made for solving such problems. As for example, there has been proposed a multilayer structure including a plurality of PWBs in which the substrates constituting the adjacent layers are bonded together through local mounds such as raised bumps formed from insulating resin, e.g., polyimide, solder balls or raising of conductive patterns so as to form an air gap between the two substrates (See, for example, U.S. Pat. No. 5,786,986). By means of the air gaps formed between the substrates, not only the dielectric loss of the high frequency circuits can be decreased, but also the occurrence of thermal stress due to a difference in linear expansion can be greatly decreased as compared with the case where shielding material having a high coefficient of linear expansion e.g., organic material is present around the bonded joints between the electrodes of the wirings, thereby ensuring improved reliability of the bonded joints as well as the improved heat dissipating properties and reduced weight of the whole assembly.
In this case, the size of the air gaps between the adjacent substrates is determined depending on the thickness of the local mounds such as the bumps, balls or raised portions. Where solder balls are used for the formation of local mounds, essentially it is difficult to make nonuniform the amount of application of solder on the same substrate and also, in the case of soldering for bonding by passing through a reflow furnace, there is the danger of the solder balls between the substrates being collapsed excessively due to the load applied to the substrates, thus making it extremely difficult from the production technical point of view to uniformly control the size of air gaps between the adjacent substrates at the desired value.
Where raised bumps of insulating resin are disposed between the adjacent substrates, the coefficient of linear expansion in the substrate thickness direction is determined by the resin forming the bumps. The coefficient of linear expansion of the raised resin is generally as high as between 50 and 150 ppm and its ill effect on the bonded joints between the substrate and electronic devices is still unavoidable. In addition, separation between the printed wiring conductor, e.g., copper and the raised resin tends to occur easily and the mechanical strength of bonded joints between the substrates is low in reliability.
It is the primary object of the present invention to provide a multilayer circuit board capable of effectively preventing the occurrence of cracking at the bonded joints between the printed wirings formed on substrates and electronic devices bonded thereon by soldering.
It is another object of the present invention to provide a multilayer circuit board formed by laminating a plurality of printed wiring boards and capable of preventing the occurrence of cracking at the bonded joints as mentioned previously, more particularly such multilayer circuit board capable of realizing bonded joints having a sufficient mechanical strength and air gaps having a stable spacing dimension between the adjacent substrates and excellent in freedom of wiring designing than conventional MPWBs as well.
In accordance with a basic concept of the present invention, a multilayer circuit board has a multilayer structure of a plurality of printed wiring boards including at least a first printed wiring board and a second printed wiring board, and each of said first and second printed wiring boards includes a metal core substrate having a first major surface and a second major surface which are opposite and parallel to each other and each of which major surfaces is covered with an electrically insulating layer, a conductive printed wiring layer formed on the surface of said electrically insulating layer, a solder resist layer covering the surface of said conductive printed wiring layer, and local bonding means for mechanically bonding together a pair of the printed wiring boards which are adjacent to each other so as to provide an air gap of a predetermined spacing value between these adjacent printed wiring boards in the multilayer structure. The metal core substrate is provided with at least one preliminarily prepared aperture extending through the first and second major surfaces to form a through-hole. The local bonding means includes a plurality of metal projections of a predetermined height which are formed on the first major surface and/or the second major surface so as to be integral with the metal core substrate and to provide an air gap between the adjacent printed wiring boards in the multilayer structure. Preferably, the metal projections are formed into a spot shape, linear shape or planar shape of a limited area. The conductive printed wiring layer formed on the electrically insulating layer includes a plurality of wiring lines on the first or second major surface and a through-hole conducting portion on the inner surface of the aperture. The solder resist layer includes a local opening made so that the metal surface is exposed at the region of the local bonding means.
In accordance with an advantageous embodiment of the present invention, each of the metal projections includes an exposed metal top of its own.
In accordance with another advantageous embodiment of the present invention, each metal projection includes an exposed metal surface layer formed by plating the metal core substrate with a metal.
In accordance with still another advantageous embodiment of the present invention, each metal projection has a top covered with an electrically insulating layer on which an exposed metal surface layer is formed by plating a metal. This exposed metal surface layer can constitute a part of the conductive printed wiring layer.
In accordance with still another advantageous embodiment of the present invention, each metal projection includes a protrusion formed from the metal core substrate by press forming.
In accordance with still another advantageous embodiment of the present invention, each metal projection includes a protrusion left after etching treatment of the metal core substrate.
In accordance with still another advantageous embodiment of the present invention, the second major surface of the first printed wiring board and the first major surface of the second printed wiring board face each other through an air gap, and the local bonding means includes a combination of a metal projection formed on one of the second major surface of the first printed wiring board and the first major surface of the second printed wiring board and a local exposed metal portion provided on the other of the second major surface of the first printing wiring board and the first major surface of the second printed wiring board a at position which is in alignment with the metal projection. This exposed metal portion can be formed from a part of the surface of the metal core substrate or a part of the conductive printed wiring layer and it can be utilized, along with the metal projection, as the required electrode for electrical connection between the adjacent printed wiring boards.
In a multilayer circuit board according to the present invention, the both surfaces of each of metal core substrates are each covered with an electrically insulating layer and individual printed wiring boards are formed by utilizing the metal core substrates each covered with the insulating layers. Thus, the structural strength of the printed wiring board is governed by the rigidity of the metal core substrate and the coefficient of linear expansion of the printed wiring board per se is substantially the same with that of the metal core substrate. Thus, if a metal material having as low a coefficient of linear expansion as that of a material used for forming semiconductor devices, e.g., silicon is selected for a metal core substrate, it is possible to prevent the occurrence of an excessive thermal stress at the soldered joint between a printed wiring board and a semiconductor device mounted thereon due to a difference in coefficient of linear expansion between the two as well as the resulting occurrence of cracking.
Preferably, if the same metal material is used as the material for the metal core substrates of a plurality of printed wiring boards forming a multilayer structure, the printed wiring boards of the respective layers become substantially equal in coefficient of linear expansion to each other so that the difference in surface-direction thermal stress among the printed wiring boards in the multilayer structure is reduced extremely with respect to the effect of heat, e.g., the heating during such production and assembling processes as a reflow process and the generation of heat in the actual use, thereby greatly reducing the occurrence of internal stress at the soldered joints between the printed wiring boards.
Particularly, in a multilayer circuit board according to the present invention, integrally formed on the first major surface and/or second major surface of each of metal core substrates are metal projections which provide bonded local joints between the respective printed wiring boards in the multilayer structure and the thickness-direction coefficient of linear expansion of the individual printed wiring boards is also determined by the metal projections, that is, by the metal material used for the metal core substrates, thereby ensuring enhanced mechanical stability against heat at the bonded joints between the printed wiring boards in the multilayer structure. In addition, the gap width of air gaps formed between the respective adjacent printed wiring boards laminated in the multilayer structure is also determined by the height of the metal projections. The metal projections can be formed preliminarily with a highly accurate projection height by etching, e.g., subtractive process or electroplating, e.g., additive process effected on one or both surfaces of each metal core substrate. Thus, in accordance with the present invention, the air gaps between the respective printed wiring boards in the multilayer structure can be formed with the desired spacing value by preliminarily controlling the height of the metal projections in contrast to those formed by solder bumps, and at the same time these air gaps can be realized as highly accurate parallel gaps by preliminarily controlling the height of the plurality of metal projections at a given value.
Also, in accordance with the multilayer circuit board of the present invention, it is possible to design so that the metal core substrates forming the printed wiring boards of the respective layers not only exhibit electric shielding performance but also serve the function of balanced strip lines as high frequency transmission lines or the function of the grounds of microstrip lines, for example. Thus, in such a case, a multilayer circuit board can be realized which makes easy the matching of circuit impedance and which is suited for use in high frequency applications.
Further, in accordance with the multilayer circuit board of the present invention, the through-hole forming apertures have been preliminarily made in the metal core substrates so that there is provided the multilayer circuit board which has no need to drill through-holes in the printed wiring boards after the formation of printed wiring patterns of electric circuitry as in the case of the prior art, which involves no danger of the delamination at the internal lamination interface of the printed wiring boards due to the shock during drilling as well as the resulting migration and the like and which is capable of highly maintaining the inter-through-hole electric insulating performance over a long period of time. In addition, due to the fact that the formation of through-holes at the desired positions is possible for each of the individual printed wiring boards forming the multilayer structure, the degree of freedom from the standpoint of wiring designing is very greatly improved over the prior art multilayer circuit boards in which through-holes are formed at once through the whole layers after the plurality of printed wiring boards have been laminated.
In accordance with the multilayer circuit board of the present invention, a part of the surface of the metal core substrates in each of the printed wiring boards can be selectively exposed so that the exposed metal portions can be utilized as bonding electrodes between the respective printed wiring boards in the multilayer structure. In this case, the adjacent printed wiring boards can be directly bonded together electrically and mechanically. As a result, the bonded joints between the printed wiring boards are high in reliability and the structural strength of the multilayer circuit board as a whole is also high. As regard the combination of the bonding electrodes, it is only necessary to combine the bonding electrodes formed on the metal projections of one of the adjacent printed wiring boards with the exposed metal portions formed selectively, the exposed metal portions formed on the metal projections or the plated electrodes formed on the insulating layer in the metal core substrate of the other printed wiring board.
It is to be noted that the metal projections integrally formed on the metal core substrate can be made to extend linearly or in a frame manner along the substrate periphery. In the later case, by forming a multilayer structure with a plurality of such printed wiring boards, the surroundings of the air gaps are enclosed with electromagnetic shielding by the metal projections forming a frame or an enclosure so that any electromagnetic interference between the electronic circuitry mounted on the printed wiring boards and the outside, that is, any undesired electromagnetic radiation from the electronic circuitry to the outside or the introduction of noise into the electronic circuitry from the outside can be effectively prevented.
The above and other features and advantages of the present invention will become more apparent from the following description of the preferred embodiments shown only for illustrative purposes without no intention of limitation of the scope of the present invention, when taken in conjunction with the accompanying drawings.